A light year is the distance light travels in a year. So, in order to see how far away an object projecting light is, we measure how long the light took to reach Earth then use that to see how far it traveled. If light from a star took 3 years to reach us, we would find that the light traveled about 18 trillion miles to reach us, since the distance light covers in a year is 6 trillion miles (a light year).
